Method Statement: Deep Excavation Temporary Support Works Near Existing Structures and Services – Method Statement

Purpose

Define the procedure for deep excavation adjacent to existing structures and live services, including temporary support works (embedded retaining walls, struts/tiebacks), groundwater management, instrumentation/monitoring, settlement control, inspection hold points, and coordination among design and construction teams.

Extent

• Excavation depth: typically 4–15 m below existing ground level [Verify per project specifications].
• Plan area: per IFC drawings.
• Proximity constraints: excavation edge within 0.5–3.0 m of existing structures/services [Verify].

Inclusions

• Pre-construction surveys, utility detection and protection/diversions.
• Temporary works design coordination and checks per BS 5975 (or equivalent).
• Installation of embedded wall system (e.g., secant pile wall, sheet pile wall, or soldier pile and lagging) per approved design.
• Staged excavation and installation of internal bracing (walers/struts) and/or ground anchors.
• Groundwater control (wellpoints/deep wells/sumps) and discharge management.
• Instrumentation and monitoring (inclinometers, settlement points, piezometers, strut load cells, building monitoring), trigger levels and response actions.
• Quality control, inspection and test plan (ITP) with hold/witness points.
• Emergency preparedness and response.
• Interface and coordination with geotechnical, structural, temporary works, and construction teams.

Exclusions

• Permanent works construction other than interfaces (e.g., capping beam/working slab where needed for temporary stability).
• Utility diversion construction beyond temporary support/protection unless specified.

Key Assumptions

• Temporary works design issued and checked by competent designer; CAT III check where required [Verify per project procedures].
• Monitoring and dewatering consents/permits in place before works commence.
• Adjacent asset owners are notified and access for monitoring is granted.

• Approved temporary works design and check certificates (CAT per project) and construction drawings.
• Temporary Works Register updated; appointments of TWC/TWS issued.
• Method Statement, Risk Assessment, and ITP approved.
• Permit-to-Dig (PTD), Excavation Permit, Lifting Plans, Hot Work Permit (if welding), Confined Space Permit (if required) [Verify per project HSE plan and local regulations].
• Utility surveys (GPR + EML) and trial pits completed, services positively identified/marked; isolation or protection agreed with owners.
• Pre-condition structural/settlement survey of adjacent assets completed with photographic record.
• Monitoring Plan approved including trigger/action levels; access arrangements with third parties secured.
• Dewatering design, discharge consent/permit, and receiving-water method approved; pump test plan approved.
• Traffic management plan and logistics routes established; laydown/stockpile areas defined.
• Baseline monitoring data collected for ≥7 days trend where practicable [Verify].
• Plant certifications, operator competencies, and lifting accessories certificates available.
• Emergency Response Plan (ERP) briefed; materials on standby: steel plates, sandbags, additional struts, generators, standby pumps.
• Environmental controls set up: silt tanks, oil-water separators, drip trays, spill kits.
• Stakeholder notification and work windows agreed (noise/vibration-sensitive receptors).

Key Hazards, Consequences, and Controls

1) Hazard: Excavation collapse/ground loss adjacent to structures
- Consequence: Serious injury/fatality, damage to adjacent buildings/services
- Controls: Design verified per BS 5975/EN 1997; staged excavation with support installed at defined levels; daily TWC inspections; no over-excavation; maintain berms; immediate installation of struts/anchors after reaching trigger level; emergency steel plates and backfill on standby
- PPE: Hard hat, boots, gloves, eye protection, high-vis
- Collective measures: Edge protection, access platforms, exclusion zones; engineered trench boxes where applicable
- Inspection/Permit: Excavation Permit; daily excavation checklist; Hold Points at each stage

2) Hazard: Underground utility strike (electric, gas, water, fiber)
- Consequence: Electrocution, explosion, flooding, service outage
- Controls: PTD with GPR/EML + trial pits; maintain 1 m no-dig buffer to live services; hand/air-vac digging in proximity; temporary protection slabs/sling supports; isolation/consents where possible
- PPE: Dielectric gloves (as required), insulated tools, arc-rated PPE for HV proximity [Verify]
- Collective measures: Barriers, warning signage, lockout/tagout where applicable
- Inspection/Permit: Permit-to-Dig; utility owner liaison approvals; standby emergency clamps/plugs for water mains

3) Hazard: Sudden water ingress/flooding due to groundwater or main burst
- Consequence: Drowning, basal heave, instability
- Controls: Dewatering with standby pumps and auto-start floats; high-level alarms; staged drawdown; base heave checks; emergency sump capacities and sandbags
- PPE: Waterproof boots, life jackets where flooding risk, gloves
- Collective measures: Safe egress ladders every 25 m; non-slip access; lighting and emergency power
- Inspection/Permit: Dewatering permit; pump maintenance logs; ERP drills

4) Hazard: Plant/crane/rig instability and lifting operations
- Consequence: Crush injury, collapse, dropped loads
- Controls: Certified cranes/rigs; outrigger mats; lift plans and dedicated AP/Lift Supervisor; wind thresholds [Verify]; taglines; exclusion zones; no lifting over live traffic/services where practicable
- PPE: Helmets with chin straps, gloves, safety footwear
- Collective measures: Barriers/spotters; radio comms; proximity alarms
- Inspection/Permit: LOLER/third-party certifications; pre-lift checks; lifting permit

5) Hazard: Working at height at excavation edges/within frames
- Consequence: Falls from height
- Controls: Edge protection (toe board, mid, top rails) or proprietary systems; MEWPs/scaffolds with design tags; harness only where collective protection not feasible
- PPE: Harness with double lanyard if required, helmet with chin strap
- Collective measures: Guardrails and platforms
- Inspection/Permit: Work at Height permit; scaffold inspections (weekly/after events)

6) Hazard: Falling objects (tools, strut components)
- Consequence: Head injury, fatality
- Controls: Tool lanyards; exclusion below; controlled lifting; netting where necessary
- PPE: Hard hat with chin strap, eye protection
- Collective measures: Overhead protection gantries where needed
- Inspection/Permit: TWC inspection before entry below

7) Hazard: Hot works (strut/waler welding)
- Consequence: Burns, fire, fumes
- Controls: Hot Work Permit; WPS/PQR; fire blankets/extinguishers; fire watch and gas testing if confined areas
- PPE: Welding shields, FR gloves/clothing, respiratory protection if needed
- Collective measures: Local extraction/ventilation
- Inspection/Permit: Hot Work Permit; equipment inspection

8) Hazard: Confined/limited access within excavation frames
- Consequence: Asphyxiation, restricted egress
- Controls: Atmosphere testing if suspected gases; rescue plan; maintain ladders at ≤25 m spacing; communication plan
- PPE: Gas detector (if required), harness for rescue, lighting
- Collective measures: Ventilation fans as required
- Inspection/Permit: Confined Space assessment/permit [Verify]

9) Hazard: Noise and vibration from piling/excavation
- Consequence: Hearing damage; nuisance; structural effects
- Controls: Select low-vibration methods where feasible; set PPV/LAeq limits [Verify]; real-time monitors; time restrictions; pre-condition surveys
- PPE: Hearing protection
- Collective measures: Acoustic screens where practicable
- Inspection/Permit: Noise/vibration monitoring records; local authority consents

10) Hazard: Traffic interface and plant-person interaction
- Consequence: Collision, injury
- Controls: One-way routes; pedestrian segregation; trained banksmen; proximity alarms; speed limits
- PPE: High-vis, safety boots, helmets
- Collective measures: Physical barriers, lighting
- Inspection/Permit: Traffic management plan; daily briefings

11) Hazard: Contaminated ground or ground gases
- Consequence: Exposure/illness, explosive atmospheres
- Controls: Review GI; gas monitoring; dust suppression; appropriate waste classification; COSHH controls
- PPE: Nitrile gloves, RPE (FFP3) as required, coveralls
- Collective measures: Decontamination area; sealed containers for spoil
- Inspection/Permit: Hazardous waste consignment notes; COSHH assessments

Note: All controls to be verified per project HSE plan and local regulations.

• Groundwater discharge: Treat via settlement tanks, silt bags, and oil-water separators. Sample TSS, pH, hydrocarbons per permit. Typical benchmark: TSS < 100 mg/L; pH 6.5–9.0 [Verify per permit].
• Drawdown management: Assess and limit drawdown radius to prevent settlement of adjacent assets. Use recharge wells if required. Monitor piezometers and settlement markers.
• Noise/vibration: Adopt low-vibration methods (press-in, pre-auger). Set site-specific limits [Verify]; typical building PPV 2–5 mm/s. Time restrictions and community notifications.
• Dust and air quality: Water sprays, wheel wash, covered stockpiles, speed control. Use atomized mist during dry/windy conditions. Monitor PM10/PM2.5 if required.
• Spoil and water management: Classify excavated materials; segregate contaminated spoil; track with waste transfer notes. Prevent runoff to drains; maintain spill kits.
• Fuel and chemical storage: Bunded areas (110% capacity), drip trays, secure taps. Refuel >10 m from drains/watercourses [Verify].
• Surface water protection: Silt curtains or barriers near receptors; lined sumps.
• Vegetation and trees: Protect root zones; avoid compaction within RPAs; install geogrids/mats for access if needed.
• Cultural heritage/archaeology: Stop-work and notify if finds encountered; follow watching brief if specified.
• Carbon/energy: Use Stage V/low-emission plant; optimize pump sizing; auto-stop systems; consolidate deliveries.

General QA/QC

• Implement ITP with clearly defined Hold Points (HP) and Witness Points (W).
• Maintain Temporary Works Register and as-built records including wall/strut/anchor layout and elevations.

Tolerances/Acceptance (typical benchmarks – Verify)

• Wall verticality: ≤1:100–1:200; plan position: ±25–50 mm.
• Toe/penetration: As design, verified by logs/survey.
• Strut preload: 80–110% of specified preload; lock-off within ±5%.
• Anchor testing: Per EN 1537; creep ≤ 2 mm at specified time at max test load; free-length elongation within ±10% theoretical; grout f_c ≥ 25–40 MPa at 28 days.
• Excavation levels: ±50 mm to each stage; base ±20 mm.
• Monitoring triggers: As per approved plan (typical settlement green <10 mm; wall deflection <0.5%H; strut force deviation <10%).
• Dewatering: Water level ≥1.0–1.5 m below base; discharge quality per permit.

Testing and Records

• Concrete/grout sampling per EN 206/EN 1537; cube/cylinder breaks at 7/28 days.
• Weld inspections per ISO 5817 quality level (C/B as specified); NDT extent typically 10–25% [Verify].
• Bolt torque/tension verification; mill certificates EN 10204 3.1 for steel; anchor tendon certificates.
• Instrumentation calibration certificates and installation records.
• Daily/weekly monitoring reports; trigger exceedance action logs.
• Non-conformance control, corrective actions, and trend reviews.

Hold Points (indicative IDs)

• HP-1: PTD and utility exposure/protection complete.
• HP-2: Instrumentation installed, baseline accepted.
• HP-3: Dewatering pump test accepted.
• HP-4: Embedded wall installation check (alignment, toe) accepted.
• HP-5: First excavation stage level/stability accepted.
• HP-6: Strut/waler pre-load accepted.
• HP-7: Each subsequent stage prior to proceeding deeper.
• HP-8: Anchor performance/proof testing accepted.
• HP-9: Base stability check accepted.
• HP-10: Approval to remove temporary supports/backfill.

• Approved temporary works design drawings and calculation packs (with check certificates).
• Staging and sequencing drawings (levels of excavation, strut/tieback schedule).
• Monitoring Plan with trigger/action levels and reporting templates.
• Dewatering design and pump test procedure; discharge permit/consent.
• Permit-to-Dig forms and utility as-built drawings; trial pit records.
• Lift plans and lifting accessories certificates.
• Welding WPS/PQR, welder qualifications, and NDT plan.
• QA/QC ITP checklists and inspection forms.
• Emergency Response Plan (ERP) including inundation/collapse scenarios, contact list, and muster points.
• Traffic management plan and site logistics layout.
• Environmental Management Plan (noise/vibration/dust/sediment control).
• Pre-condition survey report and photographic records.
• As-built survey of embedded wall, supports, and instrumentation locations.